Details of the CATV modulator are well known to persons skilled in this art and can be located from many prior documents describing the operation of these arrangements. Extensive details are available in the Technical Reports available from Cable Television Laboratories Inc and particularly the reports entitled
Data-Over Cable Service Interface Specifications Modular Headend Architecture which is CM-TR-MHA-V02-081209 copyright 2008; and
Data-Over Cable Service Interface Specifications which is CM-TR-CMAP-V01-101222 copyright 2010.
These documents are published and available on line from their web site at Cablelabs.com. The disclosure of the above documents is hereby incorporated by reference.
Present designs often employ an open-loop system of calibration or control in order to optimize the signal quality of an analog CATV or QAM output or to eliminate interfering signals from the output signal. This approach suffers from an inability to adapt to longer term effects such as aging or changes in non-linear characteristics. Aging effects are changes in the operating parameters of electronic components over time. For example, oscillators' output frequencies may drift and amplifiers' gain and distortion products may change over time.
A common application of a dedicated closed-loop feedback path in transmitter hardware is to pre-distort for the effects of amplifier non-linearity. These systems are implemented as a single feedback path per output port and do not allow for the real-time monitoring, tuning, or control of other interfering distortions or the CATV/QAM channel characteristics. In addition, these systems do not allow for the isolation of individual channel or frequency-band-specific information.